Listening to Audition 1.5 on Windows 10

I've been using AA 1.5 for ages (yes, I know, but I like it!) on Windows 7 and have now installed it on a Windows 10 laptop.  All went well and seems stable.  But I can only hear the output on the internal speakers.  It appears on the volume mixer and I can set the output to headphone (or USB interface) on advanced settings, but it still only comes out on internal speakers.  Any ideas? Thanks